LP1796903: Make date picker calendar button more accessible

LP1796903: Make date picker calendar button more accessible

To test:

1) After applying this commit, navigate to a place in the Web client
that includes a date picker (such as the checkin screen).
2) Hover over the calendar button with your mouse and confirm that a
tooltip appears.
3) Verify that the button element has an aria-label attribute.
4) Verify that the button element has an aria-pressed attribute that
is false when the date picker is closed; true when it is open.
5) Use a screen reader (like NVDA or ChromeVox).  Verify that when you
tab over to the date picker, the user is informed of the aria-label
and aria-pressed values

Signed-off-by: Jane Sandberg <sandbej@linnbenton.edu>
Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>